Wavelength division multiplexing (commonly referred to as WDM) is a technique used to increase the data-carrying capacity of an optical communication system, or of components such as an optical transmission link (e.g., an optical fiber) within an optical communication system. WDM enables an optical communication system to simultaneously carry multiple streams of data through a single optical transmission link in separate and independent optical channels, thus substantially increasing the system's data capacity. Each channel includes an optical carrier signal operating at a certain optical wavelength. In a transmitter, data is modulated onto each optical carrier signal and the separate optical signals are then multiplexed together and launched into a single transmission link.
Early WDM systems were generally “coarse” WDM systems in which the wavelength spacing between carrier signals was large; in one example of coarse WDM, one carrier signal operates at a wavelength of 850 nanometers (nm), while the next adjacent carrier signal operates at 1310 nm, for a wavelength spacing of about 460 nm. In an effort to transmit ever more data through fewer transmission links, more recent optical systems use “dense” WDM, in which the wavelength spacing between adjacent optical carrier signals is substantially smaller than in coarse WDM systems.
Cross-talk is a phenomenon that occurs in both coarse and dense WDM systems, but is especially problematic in dense WDM systems because of the small wavelength spacing. Cross-talk occurs when power is transferred between an optical carrier signal and an adjacent carrier signal. As a result of this power transfer, a small proportion of the optical power that should have ended up in a particular channel ends up in an adjacent channel, where it creates noise by distorting both the adjacent carrier signal and any data that may have been modulated onto the adjacent carrier signal.
To reduce cross-talk, current dense WDM systems use relatively large and expensive multiplexers and de-multiplexers that are temperature controlled. Coarse WDM systems are less expensive and not temperature controlled, but they allow only a very low number of wavelengths, severely limiting the aggregated bandwidth of the system. Finding alternative methods of reducing cross-talk would allow dense WDM systems to reduce wavelength spacing and would allow the use of cheaper, physically smaller and lower power consumption components.